Inventories and fluxes of ²¹⁰Pb, ¹³⁷Cs and ²⁴¹Am determined from the soils of three small catchments in Cumbria, UK

Inventories and vertical profiles of fall-out ²¹⁰Pb, ¹³⁷Cs (both weapons test and Chernobyl) and ²⁴¹Am (a decay product of ²⁴¹Pu) have been measured in the soils of three catchments in Cumbria, UK. Soil types varied from mineral soils to highly organic peat bog. The ²¹⁰Pb inventories in different cores from the same catchment were relatively uniform, the standard deviation of measurements being around 30%. Mean annual fluxes of ²¹⁰Pb were calculated from the soil inventories were 63 Bq m^-2 year^-1 (Brotherswater catchment), 63 Bq m^-2 year^-1 (Blelham Tarn catchment), and 97 Bq m^-2 year^-1 (Devoke Water catchment) per metre of rainfall. From these and earlier data published in the literature, the mean UK ²¹⁰Pb flux is estimated to be 77 ± 14 Bq m^-2 year^-1 per metre of rainfall. Inventories of ¹³⁷Cs (weapons test and Chernobyl) were more variable, but mean values for each of the catchments were in agreement with independent studies. The mean weapons test deposition (decay corrected to 1986) was 2790 Bq m^-2 per metre of rainfall, compared to UK average of 3160 Bq m^-2 derived from extensive national surveys. Although ²⁴¹Am levels in Brotherswater soils were comparable with those expected from weapons test fall-out, significantly higher values were recorded at Blelham Tarn and Devoke Water. Well-resolved peaks in ²⁴¹Am activity some distance below the present surface suggest that this radionuclide is relatively immobile within the soil column. The depth of penetration of 75% of the inventory into the soil column was similar for ²¹⁰Pb, ¹³⁷Cs (weapons) and ²⁴¹Am, all being significantly greater than Chernobyl ¹³⁷Cs. The vertical profiles of ¹³⁷Cs (weapons) suggest that there has been little mobility during the 15 years since a previous study (Cawse 1983).